CRAC Class C (2025) — Questions 411–420

U0763. Digital communication often involves the two parameters "symbol rate", which describes the change of signal states, and "data rate", which describes the information flow. The communication mode BPSK31 uses two opposite phase offsets of subcarrier f as symbols to transmit the digits 0 and 1, and the symbol transmission duration is about 32 ms. The symbol rate and data rate of BPSK31 are respectively:

数字通信经常涉及描述信号状态变化的“符号速率”和描述信息流量的“数据速率”这两个参数。通信方式BPSK31使用副载波f的两个相反的相位偏移作为符号来传输数字0和1，并且符号的发送时长约为32毫秒。BPSK31的符号速率和数据速率分别为：

Exam tips · 考试提示

Symbol rate = 1 / 0.032 s ≈ 31.25 baud. Binary PSK carries 1 bit per symbol, so data rate ≈ 31.25 bits/s (A) — this is the origin of the "31" in PSK31. The name PSK31 literally reflects its ~31 baud/bit rate.

U0764. Digital communication often involves the two parameters "symbol rate", which describes the change of signal states, and "data rate", which describes the information flow. The communication mode MFSK16 uses 16 subcarriers within a 316 Hz bandwidth as symbols to transmit 16 states, and the symbol transmission duration is 64 ms. The symbol rate and data rate of MFSK16 are respectively:

数字通信经常涉及描述信号状态变化的“符号速率”和描述信息流量的“数据速率”这两个参数。通信方式MFSK16使用 316Hz带宽内的16个副载波作为符号来传输16种状态，并且符号的发送时长为64毫秒。MFSK16的符号速率和数据速率分别为：

Exam tips · 考试提示

Symbol rate = 1 / 0.064 s = 15.625 baud. With 16 states per symbol, each symbol carries log₂(16) = 4 bits, so data rate = 15.625 × 4 = 62.5 bits/s (A). This shows how multi-level symbols boost data rate without raising baud.

U0765. Digital communication often involves the two parameters "symbol rate", which describes the change of signal states, and "data rate", which describes the information flow. The communication mode QPSK31 uses a pair of reverse phases of carrier f that are mutually orthogonal as symbols to transmit the digits 00 to 11, and the symbol transmission duration is about 32 ms. The symbol rate and data rate of QPSK31 are respectively:

数字通信经常涉及描述信号状态变化的“符号速率”和描述信息流量的“数据速率”这两个参数。通信方式QPSK31使用载波f的互为正交的一对反相相位作为符号来传输数字00至11，并且符号的发送时长约为32毫秒。QPSK31的符号速率和数据速率分别为：

Exam tips · 考试提示

Symbol rate = 1 / 0.032 s = 31.25 baud (same 31 as PSK31). QPSK uses 4 phases (00, 01, 10, 11), i.e. 2 bits per symbol, so data rate = 31.25 × 2 = 62.5 bits/s (A). The "31" refers to the baud; the doubling comes from 4-phase (2-bit) symbols.

U0766. Some commonly used amateur radio digital communication protocols only support the transmission of 5-bit or 7-bit data bytes, and are therefore unsuitable for transmitting 8-bit- byte binary data files. Among the following modes, the one that can support binary data file transmission is:

有些常用的业余无线电数字通信协议只支持5位或者7位数据字节的传送，因而不适合传输8位字节的二进制数据文件。下列方式中可以支持二进制数据文件传输的是：

Exam tips · 考试提示

PACTOR is an ARQ error-corrected link protocol built for general data, including 8-bit binary files (A). Plain RTTY, AMTOR, and PSK31 are text-oriented (5/7-bit) modes not suited to raw 8-bit binary transport (B, C, D).

U0767. Among the error-control techniques in amateur data communication there are two kinds: "forward error correction" and "automatic repeat request". Forward Error Correction (FEC) means:

业余数据通信中的差错控制技术有“前向纠错”和“自动重传请求”两种。前向纠错（Forward Error Correction，FEC）是指：

Exam tips · 考试提示

Forward Error Correction (前向纠错, FEC) adds redundant check bits so the receiver can detect and correct errors on its own without a round trip (A). B and C and D describe retransmission/repetition schemes (ARQ or brute-force repeat), not true FEC.

U0768. Among the error-control techniques in amateur data communication there are two kinds: "forward error correction" and "automatic repeat request". Automatic Repeat Request (ARQ) means:

业余数据通信中的差错控制技术有“前向纠错”和“自动重传请求”两种。自动重传请求（Automatic Repeat Request， ARQ）是指：

Exam tips · 考试提示

Automatic Repeat Request (自动重传请求, ARQ) requires a feedback link: the sender retransmits when it gets a re-request or no acknowledgment (A). B is the FEC definition; C/D are not the standard ARQ description (ARQ acts on missing ACK, not on the receiver echoing the whole message back).

U0769. The method of conducting RTTY contacts using a personal computer (PC) and an ordinary voice transceiver is:

利用个人电脑（PC）和用于话音的普通收发信机进行RTTY通联的操作方法为：

Exam tips · 考试提示

To run RTTY with a PC and voice transceiver, the sound card generates an FSK audio tone pair (the digital baseband), which an SSB transmitter shifts up to the RF carrier (A). MT63 (B) and DRM (C) are different modes, and SSB alone (D) without the FSK audio cannot produce RTTY.

U0770. The operating method for conducting PSK31 contacts using a personal computer (PC) and an ordinary voice transceiver is:

利用个人电脑（PC）和用于话音的普通收发信机进行PSK31通联的操作方法为：

Exam tips · 考试提示

PSK31 is generated by the computer as an audio signal (the PSK baseband modulation) which is then fed into the microphone/audio input of a normal SSB transceiver; the SSB stage simply shifts that audio up to the RF carrier (频率搬移). MFSK16 (B) and GMSK (C) are different digital modes, and D is wrong because PSK audio must still be SSB-modulated onto RF.

U0771. The operating method for conducting HF PACKET contacts using a personal computer (PC) and an ordinary voice transceiver is:

利用个人电脑（PC）和用于话音的普通收发信机进行短波 PACKET通联的操作方法为：

Exam tips · 考试提示

PACKET (based on the AX.25 protocol) is sent by feeding an FSK audio tone pair from the PC into the SSB transceiver; the SSB stage shifts the audio baseband up to RF (频率搬移). THROBX2 and ALE1200 are not standard PACKET modems, and D omits the FSK baseband step.

U0772. PACKET is an amateur radio communication mode developed by enthusiasts using the AX.25 packet communication protocol. When used for HF, VHF, and satellite communications, the symbol rates typically selected are:

PACKET是爱好者们利用AX.25数据分组通信协议开发的业余无线电通信方式。用于HF、VHF和卫星通信时，所用符号速率通常选定为：

Exam tips · 考试提示

PACKET/AX.25 is conventionally run at 300 baud on HF, 1200 baud on VHF (e.g., APRS), and up to 9600 baud on VHF/satellite links. These three rates (300/1200/9600) are the standard choices; the distractors mix in non-standard speeds.